Many high temperature applications require a material that is not only processable into a fibrous structure but is also capable of withstanding severe end-use temperatures. In some instances, these temperatures may be as high as 1000 degrees Celsius to 2000 degrees Celsius. The existing engineering plastics cannot be used in such applications because most plastics decompose below 1000 degrees Celsius. Moreover, such plastics suffer dramatic losses in mechanical properties such as tensile strength and tenacity at temperatures as high as 250-400 degrees Celsius. For example, KEVLAR 29 (a trademark of DuPont), when heated to 250 degrees Celsius in air can lose 60% of its tenacity and 60% of its tensile strength. At 425 degrees Celsius Kevlar irreversibly degrades and at 500 degrees Celsius Kevlar decomposes. NOMEX (a trademark of DuPont) decomposes at 370 degrees Celsius and polybenzyimidazole (PBI) decomposes at 480 degrees Celsius. At 520 degrees Celsius, the carbonaceous fibers of the present invention, retain 90% of their original weight.
Heretofore, ceramic, graphite fiber and quartz battings and fabrics have been used for high temperature thermal insulation and high temperature protection. All of these prior art materials are very brittle and tend to pack with time and lose loft, thus losing performance with time. The quartz and ceramic materials are air stable at high temperatures such as greater than 450 degrees Celsius. However, they are very difficult for workers to handle and present health risks to the workers similar to those problems created by handling asbestos. A significant amount of research has been conducted by industry to find fibrous materials which can be readily processed into textile batting structures or fabrics and which will withstand temperatures of 400 degrees Celsius or greater in air without loss of mechanical properties. These fibers include Celanese's PBI and Oxidized Polyacrylonitrile Fiber. While these materials are readily processable and have a high degree of resiliency, they lack the requisite thermal stability to withstand temperatures of greater than 400 degrees Celsius and still maintain good mechanical properties.
It is understood herein that the fiber assembly of the invention may be in the form of a monofilament fiber, a multifilament fiber tow, a yarn, a multiplicity of fibers forming a wool-like material, a nonwoven fiber batting, matting, webbing or felt, a woven fabric or knitted cloth, or the like.
The non-graphitic carbonaceous fibers have a carbon content of at least 65% and an LOI value of greater than 40 when the fibers are tested according to the test method of ASTM D2863-77. The test method is also known as "Oxygen Index" or "Limited Oxygen Index" (LOI). With this procedure, the concentration of oxygen in O.sub.2 /N.sub.2 mixtures is determined at which a vertically mounted specimen is ignited at its upper end and just (barely) continues to burn. The width of the specimen is from 0.65 to 0.3 cm with a length of from 7 to 15 cm. The LOI value is calculated according to the equation: ##EQU1##